This invention relates to light emitting devices and, more particularly, to semiconductor heterostructure light emitters.
Various types of semiconductor light emitters are described in the prior art. For example, in a gallium arsenide homojunction light emitter, electrons are injected across a pn junction, combine with holes, and give up excess energy by emitting light at a wavelength characteristic of the material. In a so-called double heterojunction light emitter, fabricated, for example, using a gallium arsenide/aluminium gallium arsenide material system, a pair of relatively wide bandgap layers (aluminum gallium arsenide) of opposite conductivity type are sandwiched around an active region (gallium arsenide). The interfaces between the active region and the wide bandgap layers form a pair of heterojunctions. These heterojunctions effectively provide carrier confinement and optical confinement. The devices may be used as light emitting diodes or lasers, and may be energized using an electrical current or optical pumping.
There are a number of practical constraints which affect operation and performance of semiconductor light emitting devices. For example, relatively high current densities may be necessary to achieve a desired level of light emission or laser action. Temperature is a significant consideration and, while it is desirable to have devices that work at room temperature, lower temperature operation may be required if continuous operation is desired. At room temperature, pulsed operation is typically necessary since continuous operation can cause overheating. The wavelength of the radiation produced is also significant, and is not generally a matter of flexible choice. The wavelength of radiation generated by conventional double heterojunction devices is a function of the bandgap of the active region. Within limits, the wavelength of the radiation produced can be changed by altering the composition of the active region.
In the U.S. Pat. No. 3,982,207, there is disclosed a heterostructure laser having a multi-layered semi-conductor body which includes an active region having a plurality of thin narrow bandgap active layers interleaved with a plurality of thin relatively wider bandgap passive layers. The passive layers are thin enough (indicated as being less than about 500 Angstroms) to permit carriers to distribute themselves among the active layers when the body is pumped (either optically or electrically). The distribution of carriers is stated to occur either by tunneling through, or by hopping over, the energy barriers created by the passive layers. The active layers are thin enough (indicated as being less than about 500 Angstroms and preferably about 50 Angstroms) to separate the discrete energy levels of confined carriers. In this manner, the patent indicates that quantum size effects are exploited to produce wavelength tuneability without having to rely on changes of the composition of the active region, and also to achieve lower lasing thresholds. The exceedingly thin layers of this patent are indicated as being made using molecular beam epitaxy techniques.
It is an object of the present invention to provide improved semiconductor heterostructure devices which exploit quantum size effects in a novel manner to achieve operational advantages in various applications.